When handling local files , should I call flush() at all?

Question

When handling local files, I am confused by the flush() method.
A lot of tutorial pages say it's safer to call flush() before close(). After thinking for a while, I'm asking mysel: Does flush() applies only to buffered stream(right?)? If a stream is not buffered, program should not call flush(). For buffered stream, the corresponding close() method of the wrapper or stream class should invoke flush() method. If they don't , it's an implementation bug, which should be fixed. So for both buffered and unbuffered stream, no need to call flush() before close()(right?).
Then the question is: when do we need to call flush() method? If we don't need call flush() at all, why are there flush() methods in almost every stream/wrapper class?
Please let me know what is wrong, either me or something else.
Any feedback is appreciated.
Gang

Answer 1

"The close method of FilterOutputStream calls its flush method, and then calls the close method of its underlying output stream." (Javadoc)
By inheritance this also applies to BufferedOutputStream and its descendants. However FileOutputStream does not extend FOS or BOS so it has nothing to flush(), so flushing it does nothing at any time even before closing.
In general flush() writes buffered bytes out of the JVM into the operating system. When you do have buffering, which in practice you should (e.g. new BufferedOutputStream(new FileOutputStream(...))), you should flush at points when you really want to be rid of the data. When operating on sockets you should always flush before try to read the next data from the socket. You can flush more often if you like but it is often counter-productive.